The incorporation of particles suspended in the electrolyte into plasma electrolytic oxidation coatings on Ti and Al substrates

This investigation concerns the mechanisms by which (fine) particles become incorporated into plasma electrolytic oxidation (PEO) coatings when added to the electrolyte. Three different types of particle have been used, covering a wide size range, and processing has been carried out with both Al and Ti substrates. For some of these combinations, the particulate was chemically similar to the expected PEO product, while for others it was different. The power supply was 50 Hz AC, with a pre-selected current density. It has been established that, where such reactions are chemically favoured, phase changes can occur that must have involved the particulate reaching very high temperatures. From this and other evidence, it is concluded that the main incorporation mechanism involved is that of (fine) particulate being swept into the pores associated with active discharge sites, while they are being refilled with electrolyte immediately after collapse of the plasma. They are then likely to become entrapped, and in many cases to be strongly heated as the plasma is created during the next discharge cycle. Typical pore sizes are such that particles (or particulate clusters) above about 10 mu m in size would be unlikely to enter them. While particles a few microns in diameter can become incorporated, it takes place more readily with sub-micron particles. It is also concluded that electrophoretic forces are unlikely to play any significant role in the incorporation process.